"Nobody has a strong belief that the peace conditions which were signed in Minsk will be implemented strictly," Poroshenko said.

G7 countries said they were ready to take "appropriate measures" against those who violate the agreement, an apparent threat of more economic sanctions against Russia.

In a joint statement issued by the French presidency, the G7 leaders said they remained concerned about the situation in eastern Ukraine and in particular about fighting around the town of Debaltseve ahead of the February 15 cease-fire.

"All parties should refrain from actions in the coming days that would hinder the start of the ceasefire," the statement said.

Ukrainian military spokesman Andriy Lysenko said 11 soldiers had been killed and 40 wounded in the past 24 hours.

"Rebels are repeatedly storming the strongholds and base camp of Ukrainian forces," in and around Debaltseve as well as firing artillery, mortars and rockets, Lysenko said, stressing that government troops had held their positions.

Psaki also accused Russia of backing out of another key part of the agreement — a pledge by all sides to release illegally held prisoners and hostages.

Psaki said that Nadia Savchenko, a Ukrainian air force officer imprisoned in Moscow and charged in the deaths of two Russian journalists in east Ukraine, would not be freed.

"She's a hostage of the Russian government and she must be released immediately," Psaki said.

"This week's agreement calls for the exchange of all hostages and illegally held persons taken during the conflict. In our view, this clearly includes Nadia Savchenko."

Kremlin spokesman Dmitry Peskov said that Russia had not promised to free Savchenko.